The present invention concerns a light emitting and guiding device with an active region based on silicon, and processes for manufacturing such a device.
An active region is taken to mean a region of the device in which the light is generated and/or guided before leaving the device.
The invention finds applications in the manufacture of optical or optoelectronic components such as electroluminescent diodes, lasers, or possibly photodetectors.
A particularly advantageous application of the invention, linked to the use of silicon for the active region, is the manufacture of integrated circuits that combine both electronic components and optical components. Electronic components are in fact mainly manufactured from silicon, due to the intrinsic qualities of this semi-conductive material, and due to the widespread development of technologies relating to its applications.
As evoked above, silicon is very widely employed in the manufacture of electronic components or integrated circuits using semi-conductors.
However, in certain applications, in which components intended for light emission are used, silicon turns out to be unsuitable.
In fact, silicon is a semi-conductor with an indirect forbidden band and is not suitable for the rapid recombination of carriers, in other words, electron-hole pairs, with the production of light. When the elgctrons and the holes are brought together, for example when directly polarising a p-n junction formed in the silicon, their average recombination time can reach periods of several microseconds or even longer. In fact, the phenomenon of carrier recombination is dominated by other processes that are more rapid than the radiative recombination. These processes essentially correspond to the non-radiative recombination of the carriers on defects and impurities.
The defects and impurities play an important role, even if their concentration is low. The carriers move in the semi-conductor over a large distance and the probability of their encountering a defect or impurity is high.
Thus, in a certain number of applications, silicon must be replaced by another semi-conductive material with a direct forbidden band such as, for example, gallium arsenide (GaAs). As an indication, for this semi-conductor, the average recombination time of the electron-hole pairs is around one nanosecond.
Gallium arsenide is however an expensive material and more complex to implement.
In a certain number of specific cases, and in specific conditions of use, silicon has been proposed for making light emitting or conducting devices. Examples of such uses of silicon are proposed, in particular, in documents (1) to (7), whose references are detailed at the end of the present description.
The documents propose techniques that make it possible to increase the efficiency of light emission by silicon. Nevertheless, these techniques are not generally suited to the requirements of the integration of components.
In a more specific manner, documents (6) and (7) describe light emitting or conducting devices. Examples of such uses of silicon are proposed, in particular, in documents (1) to (9), whose references are detailed at the end of the present description.
The documents propose techniques that make it possible to increase the efficiency of light emission by silicon. Nevertheless, these techniques are not generally suited to the requirements of the integration of components.
In a more specific manner, documents (6) and (7) describe light emitting devices made on a silicon type substrate over an insulator (SOIxe2x80x94Silicon an Insulator), increasingly used in the micro-electronics field. However, the low temperature operating conditions and the isotropic character of the light emission of the devices also constitute obstacles to their use as components in circuits.
Documents (8) and (9) describe, respectively, a photon resonator and particular embodiments of silicon diodes doped with erbium.
The aim of the present invention is to propose a device capable of emitting but also guiding light, which is based on silicon and which can be manufactured according to common techniques specific to the micro-electronics field.
Another aim is to propose such a device that can be used as an individual component or as an integrated component in a circuit, in association with other optical or electronic components.
Another aim is to propose such a device with an improved light emission output and capable of operating at ambient temperature.
Another aim is to propose manufacturing processes for a device according to the invention.
In order to achieve these aims, the objective of the invention is more precisely an emitting and guiding device as defined in claim 1. Claims 2 to 16 indicate particular embodiments of the device.
The device described in the invention has the advantage of both confining the carriers within a restricted region, the active region, in such a way as to reduce the probability of the carriers encountering non-radiative centres, and the advantage of offering the carriers, in this region, radiative centres with a short life time.
A short life time is taken to mean a life time shorter than the life time linked to the probability of non-radiative recombination on defects or impurities in the active region.
The active region is, for example, a thin, continuous film of silicon stacked between the first and second insulator layers. This film is preferably mono-crystalline, which gives it better radiative qualities.
According to a particular embodiment of the device described in the invention, the means used to confine the carriers comprise the first and second insulator layers and the whole assembly, comprising the active region and the insulator layers, has an optical thickness e, whereby:   e  =      k    ⁢          λ      2      
and where k is a natural integer.
In this particular embodiment, adapted to a device operating at a given wavelength xcex, the light is confined in the active region. It propagates in the principal plane of this region, particularly in the case where the active region is a thin layer of silicon, by total reflection on the insulator layers. The principal plane is defined as a plane of the active region more or less parallel to that of the insulator layers.
The total reflection is obtained thanks to an appropriate step index between the material in the active region (Si) and the material used for the insulator layers (for example SiO2).
The optical thickness e of the layer or the active region in silicon is adapted to the working wavelength xcex in such a way that: e=kxcex/2, where k is an integral number.
According to a variant of the invention, the propagation can also be allowed to be perpendicular to the principal plane. In this case, the device can, moreover, comprise the means of reflecting the light comprising at least one mirror arranged on a free face of at least one of the first and second insulator layers.
More precisely, the means of reflecting the light can comprise a first mirror arranged on the free face of the first insulator layer and a second mirror arranged on the free face of the second insulator layer, with the first and second mirrors having different transmission coefficients.
The mirror with the highest transmission coefficient can then be used as a light exit mirror.
Moreover, the first and second mirrors can form a Fabry-pxc3xa9rot type cavity with the active region.
It should be pointed out that the means of reflection also have a function of guiding the light.
As indicated previously, the active region contains radiative centres, in other words, centres that allow the radiative recombination of the carriers.
Different types of radiative centres can be used and may possibly be combined in the active region.
A first type of radiative centre can be formed from the ions of rare earth elements, possibly accompanied by other impurities.
The rare earth elements, such as, for example, erbium, praseodymium or neodymium are efficient radiative recombination centres. The wavelength of the emitted light is mainly determined by the nature of the rare earth element and only to a very small extent by the matrix, in other words, silicon in this case. The rare earth elements mentioned above are particularly interesting because their emission corresponds to wavelengths that are useful for fibre optic telecommunications (1.3 and 1.54 microns). Co-doping with other impurities such as oxygen, carbon, nitrogen or fluorine can even increase this emission in a significant manner.
A second type of radiative centre can be formed by a quantum well or a succession of several quantum wells formed by thin layers of germanium or a Si1xe2x88x92xGex alloy (where 0 less than Xxe2x89xa61)or SiGeC or any other compound suitable for the formation of quantum wells. The thickness of the layers forming the wells can, for example, be around 5 nm. In addition, the total thickness of these layers is preferably kept below a critical thickness corresponding to the appearance of lattice mismatch dislocations in the silicon. In this way, the crystalline quality in the active region remains very good. The quantum wells lead to increased radiative recombination efficiency. Moreover, a succession of wells and very thin barriers can lead to the formation of a band structure similar to a direct forbidden band structure, with a high probability of radiative recombination. Barriers of potential are formed by the silicon between the layers of germanium.
A third type of radiative centre can be formed from quantum boxes formed from a film of germanium or silicon-germanium, or from other elements introduced into the silicon.
In fact, due to the lattice mismatch parameter between the silicon and the germanium, the film of germanium, if its exceeds a thickness of several single layers, naturally transforms into a succession of isolated islands spread out over the active region, which form the quantum boxes.
These islands are between 100 and 1000 times more efficient for the emission of light than germanium in the form of a solid layer. The introduction of such islands into the silicon film thus makes it possible to form a very efficient light emitter.
The photons in the active region can be created by optical or electric xe2x80x9cpumpingxe2x80x9d.
In the first case, the means for creating the photons in the active region can comprise an additional light source.
In the case of electric pumping, the means for creating the photons in the active region comprise a diode formed in the active region.
The device of the invention can comprise a single active zone in the form of, for example, as evoked above, a continuous layer of silicon.
According to a variant, however, the device can also comprise a plurality of active regions between the first and second insulator layers and separated from each other by an insulator material.
The active regions can, for example, be islands of silicon surrounded by silicon oxide. The islands have preferably a characteristic size of between 100 and 200 nm. In particular, it can be advantageous to have a characteristic size more or less equal to the thickness.
The role of the layers or zones of insulator that delimit or surround the active zones is essentially to limit the movement of the carriers which move predominantly under the effect of diffusion, or, depending on the case, under the effect of an electrical field.
The insulator layers or zones are preferably made out of a material with a wide forbidden band, in such a way as to put high potential barriers in the way of the carriers. Amongst these insulator materials, one can cite, for example, SiC (silicon carbide).
As is the case with many other insulators, the lattice mismatch between silicon carbide and silicon is too important to envisage building up silicon on the SiC with a good crystalline quality.
In order to overcome this difficulty, the invention also has as an aim a manufacturing process for a device as described previously. This process comprises the transfer, by molecular bonding, of a thin layer of silicon, which is intended to form the active region, onto a support that forms or comprises a first layer of insulator. The process is completed by covering the said layer of silicon by a second layer of insulator.
The transfer by molecular bonding makes it possible to combine the silicon with insulator materials that have excellent potential barrier properties, even if these materials do not have a crystalline structure or if these materials have a crystalline structure that is incompatible with the growth of silicon.
The thin layer of silicon can be transferred onto a solid insulator support made out of a material such as, for example, SiC, ZnO, AlN or BN alloys.
The techniques of molecular bonding, which are well known in themselves, are not described here.
According to another possibility, the thin layer of silicon can also be bonded onto a support formed from a silicon substrate covered by a superficial layer of dielectric material commonly used in the micro-electronics field, such as SiO2, Si3N4 or quartz.
According to another particular embodiment of the process, the transfer of the thin layer comprises molecular bonding onto the support of a thick block of silicon by a transfer face, the block of silicon having a preferential cleavage zone that is parallel to the face of the transfer and which delimits the thin layer and then, after bonding, the cleavage of the said block to separate the thin layer from the block. The cleavage zone can be formed, for example, before the transfer by implanting the appropriate ions, such as hydrogen ions, at a defined depth in the block of silicon in order to create, in the block, the preferential cleavage zone.
According to a variant, the transfer of the thin layer can comprise the bonding onto the support of a thin layer of silicon connected to a substrate of the support (for example, in solid silicon) via a sacrificial layer, then the separation of the thin layer from the substrate by dissolution of the sacrificial layer.
As an example, the layer of silicon that is intended to form the active layer can be formed by epitaxy on the substrate of silicon that has been previously covered with a sacrificial layer of GeSi alloy.
The composition and thickness of the layer of GeSi are preferably chosen in such a way as to allow the growth of a layer of silicon with good crystalline quality.
The layer of silicon can also be partially oxidised in order to cover it with a layer of oxide.
The substrate with the layer of silicon is then Transferred onto the support, and the SiGe alloy is dissolved in order to detach the layer of silicon forming the active region from the substrate.
According to another possible embodiment of the device, the process can also comprise the formation of a layer of oxide buried in a block of silicon, in such a way as to delimit in it a thin superficial layer of silicon, the thin superficial layer being intended to form the active region, then covering the thin layer by a layer of oxide.